1. FIELD OF THE INVENTION
The present invention relates to an optical information recording apparatus for optically recording digital information on a recording medium, for example, an optical disc, a tape and so on.
2. PRIOR ART
Optical information recording apparatus for optically recording information on a recording medium, for example, a video disc and a compact disc is widely used in the information memory.
Informations are recorded by means of pits which are concaved on the surface of an optical disc in such a manner that the length and the width of the pits vary in accordance with the informations. Reproducing of information from the optical disc having pits recorded thereon is achieved by irradiating a laser beam onto the optical disc to form optical spots each converged by an objective lens and detecting the amount of reflection of the laser beam from the surface of the optical disc. When pits are not irradiated by a laser beam, the laser beam reflected from the optical disc returns to the objective lens without a loss, and is received to detect a photocurrent by photo detectors. However, when pits are irradiated by a laser beam, the laser beam reflected from the optical disc expands of diffraction at pits and light amount returning to the objective lens decreases. Consequently photocurrent decreases in comparison with the case that pits are not irradiated by the laser beam. The level of the recorded signals reproduced based on the photocurrent in compared with a threshold level so that digital codes are determined depending on the level of the recorded signal higher or lower than the threshold level. Thus the pits are detected by irradiating the laser beam onto the optical disc so that the recorded information on the optical disc can be reproduced.
The principle of recording/reproducing of optical information recording/reproducing apparatus is described in U.S. Pat. No. 4,558,441, in which a peak detector provided in a data recording/reproducing apparatus detects the peaks of an electrical signal and generates a pulse signal having pulses synchronous with the peaks of the electrical signal.
However, the area of optical spot can not be made smaller than a minimum area determined by a numerical aperture of the optical lens and the wave length of a reproducing beam. Consequently when the length of a pit is too short, a problem occurs that the photocurrent of the reproducing beam at the leading edge and the trailing edge of the pits does not level down sufficiently, so that the signal reproduced from the pits has a pit length information shorter than the actual length of the pit. Similarly when pit-interval is too short, the signal reproduced from the pit-interval has a pit length information shorter than the actual length of the pit-interval. This causes a problem of code discrimination failure in reproducing short pits and/or short pit-intervals. Generally, in the case where digital codes are determined by presence and absence of pits, a similar problem occurs, and whereby the information can not be recorded/reproduced on and from optical discs in high density.